Tire components such as a tread, inner liner, and sidewall usually include natural rubber and carbon black. Since these materials easily cause scorching, the incorporation of large amounts of these materials may easily cause compound scorch during the manufacturing process. The above tire components may also include an alkylphenol-sulfur chloride condensate, which also easily causes scorching, and thus have the problem of compound scorch.
Compound scorch can be prevented by, for example, a method of kneading the materials at a temperature of not higher than 140° C. or a method of extruding a rubber compound at a temperature of not higher than 120° C. Unfortunately, these methods cause poor dispersibility of filler and poor productivity.
Compound scorch can also be prevented by retarding the curing rate through the addition of wet silica with hydroxy groups or the reduced use of sulfur; however, these methods may decrease abrasion resistance, hardness (handling stability), tensile strength at break, and other properties. There is also a method of adding DCBS as a vulcanization accelerator, but DCBS is designated as one of the chemical substances subject to type I monitoring and is thus undesirable in view of environmental and other impacts.
Other known methods for retarding the curing rate include a method of adding a vulcanization retarder such as PVI. Unfortunately, the use of such a retarder in an amount of 0.5 parts or more may cause blooming during the rubber processing or after vulcanization. Additionally, when the quality varies among raw materials or under a condition where compound scorch is likely to occur (e.g., during the extremely hot season), PVI is usually added at the discretion of an on-site operator. In view of such circumstances, it is desirable to set the design center value of the PVI content of each tire component to zero.
Patent Literature 1 suggests a rubber composition for a tire cord topping containing natural rubber, a specific vulcanization accelerator and other materials. However, there is still room for improvement in terms of providing a pneumatic tire that can be produced at a good curing rate and, at the same time, provides improved processability, handling stability, fuel economy, and tensile strength at break in a balanced manner.